Enjoy this add-on while it lasts. Firefox 57 will no longer support this kind of add-on, as only WebExtensions add-ons will continue to work. This add-on is not doable with WebExtensions for (at least) two reasons:

There seems to be no way of accessing the installed search engines at all.

The WebExtensions API only allows putting a regular old menu item into the context menu. That means it's not possible to put multiple search engines in one row.